Controlling transmission mode on basis of power in preceding time slot

ABSTRACT

A transmitter  2  comprises a power amplifier  3  and power controller  4 . The power amplifier  3  is capable of both linear and non-linear operation, and is controlled by a controller  20  and first and second control loops  21,22 . The second control loop  22  is used with the transmission of GMSK modulated signals, which have no amplitude modulation, operating the power amplifier  3  in a non-linear mode. The first control loop  21  is used for the transmission of EDGE modulated signals, which have a substantial amount of amplitude modulation, operating the power amplifier  3  in a linear mode. The first control loop  21  is also used with GMSK time slots if the output power to be produced by the power amplifier  3  is low and the power of an immediately preceding time slot is high. In this case, a sample and hold circuit  9, 10  of the first control loop  21  is used when the power detector voltage in the power amplifier would change over the time slot by cooling, avoiding the possibility of the power level varying over the time slot.

FIELD OF THE INVENTION

The invention relates to a transmitter, and a method of operating atransmitter. It particularly relates to a transmitter in which data ismodulated using one of a first modulation scheme and a second modulationscheme in each of a series of time slots, and to a method comprisingtransmitting data in a series of time slots, and applying one of a firstmodulation scheme and a second modulation scheme to data in each timeslot.

BACKGROUND OF THE INVENTION

FIG. 1 depicts a communication device 1 arranged to transmit data in aseries of time slots, such as is used in TDMA systems. The communicationdevice 1 comprises a transmitter 2 that includes a power amplifier 3 anda power controller 4 arranged to control the power amplifier 3, togetherwith a RX-TX switch 5 and an antenna 6. The power amplifier 3 suppliespower to the antenna 7. A controller 20 is operable to control all theother components. The RX-TX switch 5 connects the antenna 6 to either areceiver (not shown) or the transmitter 2.

Recently, communication devices that are capable of transmitting datausing more than one modulation scheme have become available. Forexample, the Nokia (RTM) 9500 Communicator is arranged to send datausing EDGE and using GMSK (Gaussian Minimum Shift Keying) modulation.FIG. 2 depicts part of a prior amplifier arrangement 2 suitable for usein such a communication device. FIG. 2 shows a multimode power amplifier3 and components of the power controller 4.

When modulated signals with a high degree of amplitude modulation, suchas EDGE signals, are to be transmitted, the power amplifier 3 isoperated in a linear mode. The power amplifier 3 is controlled using afirst control loop 21, comprising a differential amplifier 7 a, having afeedback loop including a capacitor 8 a, and a sample and hold circuitcomprising a capacitor 9 and differential amplifier 10. The firstcontrol loop 21 controls the output power of the power amplifier 3 byaltering an input signal fed to the power amplifier 3, by adjusting thegain of a variable gain amplifier 11 preceding the power amplifier 3.The power amplifier 3 and the variable gain amplifier 11 cooperate toconstitute together a power amplifier.

A switch 12 between the amplifier 7 a and the amplifier 10 is closedduring the beginning and end of a time slot used for lineartransmission, to allow the output power of the power amplifier to beramped up or down under the control of an input ramp signal TXC. Duringthe intervening portion of the time slot, which is used for datatransmission, the switch 12 is open, thereby disconnecting thedifferential amplifier 7 a and its feedback loop. The variable gainamplifier 11 is then controlled by an output of the sample and holdcircuit 9, 10, which applies a constant control voltage to the variablegain amplifier.

At the end of a time slot, the transmitter 2 is ramped down. The powercontroller 4 can then switch between the two control loops 21,22 bymeans of switches 13 a, 13 b, 13 c, before the transmitter 2 is rampedup at the beginning of the next time slot. The ramping is performedusing a ramp signal TXC that is input to the relevant control loop inorder to control the output power of the power amplifier 3.

For modulated signals with little or no amplitude modulation, such asGMSK modulated signals, the power amplifier 3 is operated in anon-linear mode in order to improve its power efficiency. When thetransmitter 2 is provided in a communication device 1 such as a mobiletelephone, this increased efficiency may result in longer talktime. Asecond control loop 22 is provided, which comprises a differentialamplifier 7 b and a feedback loop including a capacitor 8 b but does notinclude a sample and hold circuit. The second control loop 22 controlsthe power amplifier 3 by altering a voltage applied to a power controlpin Vpctrl.

At the start and end of a time slot in which GMSK or similar modulatedsignals are to be transmitted, the output power of the power amplifier 3is ramped up or down under the control of the input ramp signal TXC. Thesecond control loop remains closed during the beginning, the interveningportion and end of the time slot. This means that, unlike the firstcontrol loop 21, the second control loop 22 remains connected to a diodepower detector 14 that forms part of and monitors the output power ofthe power amplifier 3, and adjusts the control voltage accordinglythroughout the time slot.

During a time slot in which the output power of the power amplifier 3used for data transmission is high, the temperature of the components ofthe power amplifier 3 increases. Heat from the components is transferredto the power detector 14 and may result in a decrease in the accuracy ofits measurements. The resulting measurement error may be of the order of2 mV per degree centigrade, which may result in measurement results thatare erroneously low.

If, then, during the next time slot, the power amplifier 3 is to beoperated in a non-linear mode to provide an output power that issubstantially lower than in the preceding time slot, the components ofthe power amplifier 3 and the power detector 14 will then cool downduring this time slot. This temperature drift causes a drift on thepower detector voltage during the time slot and results in the controlvoltage and, therefore, the output power of the power amplifier 3,decreasing during the intervening portion of the time slot. This isundesirable.

FIG. 3 is a graph showing the output of the power detector 14 during twosuccessive time slots t₁, t₂. In the first time slot, the output poweris high and is above the upper limit of the y-axis of the graph. Duringthe second time slot, the temperature of the power detector 14 is high,due to heat generated by the components of the power amplifier 3 duringthe first time slot t₁, and so the output of the power detector 14starts at a relatively low value, compared with the actual output powerof the power amplifier 3. The output of the power detector 14 thendrifts upwards during the second time slot t₂ as its diode cools down.For GSM900, the output power in any given time slot must fall within arange of 5 to 33 dBm, with an accuracy of +/−1 dB during the useful partof a burst. The temperature drift between time slots is most significantwhen the output power is 33 dBm during a first time slot and 5 dBmduring the succeeding time slot. FIG. 4 is a graph of the output powerover two GSM 1900 time slots, also labelled t₁, t₂, and clearly shows adownwards drift in the level of the output power during the second timeslot t₂.

In some prior amplifier arrangements, this problem has been addressed byproviding temperature compensation means for the power detector 14.However, this requires the provision of extra components. Furthermore,the provision of temperature compensation means that are capable ofresponding to very rapid temperature changes is not straightforward. Thepower detector 14 may also be located away from the power amplifier 3,for example, as in the arrangement disclosed in U.S. Pat. No. 6,369,635,so that it is not significantly affected by heat from the poweramplifier components. However, this requires the provision of externalcomponents and may increase the size of the amplifier arrangement 2.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided atransmitter arranged to transmit data modulated using one of a firstmodulation scheme and a second modulation scheme in each of a series oftime slots, the transmitter comprising:

-   -   a power amplifier operable in linear and non-linear modes;    -   power control means, arranged to control the power supplied by        the power amplifier; and    -   a controller arranged to control the power amplifier to operate        in the linear mode in time slots when the first modulation        scheme is applied, and to operate in the non-linear mode in time        slots when the second modulation scheme is applied,    -   the controller being arranged, in respect of a time slot in        which the second modulation scheme is used, to determine whether        a transmission power in the time slot is lower than the        transmission power in an immediately preceding time slot, and in        response to a positive determination to control the power        amplifier to operate in the linear mode during the time slot.

This can allow the effects of a power detector which is temperaturesensitive to be ameliorated.

Typically, the amplifier will be operable in a linear mode when thefirst modulation scheme has a significant amplitude modulation content,as is found for example with the modulation used with EDGEtransmissions. A non-linear mode, which is more efficient, typically isused where the second modulation scheme has a generally constantamplitude, for example the modulation used with GMSK.

Preferably the transmitter includes a sample and hold circuit operableto maintain the gain of the power amplifier at a generally constantvalue between ramp up and ramp down portions of a time slot whenoperating in a linear mode.

This sample and hold circuit is a particularly effective way to preventoutput power changing over a time slot. Using the sample and holdcircuit in the linear mode and operating the power amplifier in thelinear mode when the non-linear mode would normally be used, but becausethe transmission power in the time slot and in the immediately precedingtime slot would otherwise result in the power varying over the timeslot, has two consequences. Firstly, it prevents the power varying overthat time slot. Also though, since amplifiers operating in linear modesconventionally are less efficient, a result is that the poweramplification is less efficient, so more power is used for the sameoutput power. The applicant considers that this is an acceptabledisadvantage in light of the positive effects of this, namely obtaininga non-varying output power over the time slot.

The advantages of the invention are particularly experienced if thepower amplifier comprises first and second amplifier stages, an outputof one amplifier stage being coupled to an input of the other stage, inwhich in the non-linear mode the first amplifier stage is controlled tohave a fixed gain and the second amplifier stage is controlled to have again dependent on a second amplifier stage control signal generated by afirst control loop, and in which in the linear mode the second amplifierstage is controlled to have a fixed gain and the first amplifier stageis controlled to have a gain dependent on a first amplifier stagecontrol signal generated by a second control loop

Here, if the second control loop includes a sample and hold circuitoperable to maintain the first amplifier stage control signal at agenerally constant value between ramp up and ramp down portions of atime slot, this can be used to advantage both when transmitting datamodulated with the first modulating scheme and when transmitting data ina time slot when power variation might be expected if the amplifier wereoperated in a non-linear mode.

If the transmitter comprises a power detector integrated with the poweramplifier, it does not need to be provided with quickly actingtemperature compensation means. Use of an integrated power detector ispreferred since such devices are smaller, and also usually lessexpensive, than corresponding external arrangements.

According to a second aspect of the invention, there is provided amethod of operating a transmitter, the method comprising:

-   -   transmitting data in a series of time slots,    -   applying one of a first modulation scheme and a second        modulation scheme to data in each time slot; and    -   controlling a power amplifier to operate in a linear mode in        time slots when the first modulation scheme is applied, and to        operate in a non-linear mode in time slots when the second        modulation scheme is applied, the method further comprising, in        respect of a time slot in which the second modulation scheme is        used, determining whether a transmission power in the time slot        is lower than a transmission power in an immediately preceding        time slot, and in response to a positive determination        controlling the power amplifier to operate in the linear mode        during the time slot.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described with reference tothe accompanying drawings, in which:

FIG. 1 depicts a transmitter that may be operated according to thepresent invention;

FIG. 2 depicts an amplifier arrangement forming part of the FIG. 1transmitter;

FIG. 3 is a graph showing the output power over two successive timeslots; and

FIG. 4 is a graph showing an output of a power detector over twosuccessive time slots.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an embodiment of the invention, a transmitter 2 comprises a poweramplifier 3 and power controller 4, as shown in FIGS. 1 and 2. The poweramplifier 3 is capable of both linear and non-linear operation, and iscontrolled by a controller 20 and first and second control loops 21, 22as described above in relation to the prior art.

However, instead of using the first control loop 21 exclusively for timeslots in which data signals with a high degree of amplitude modulationare to be transmitted, the first control loop 21 is also used whennon-amplitude modulated signals are transmitted if the output power tobe produced by the power amplifier 3 is low and in the immediatelypreceding time slot a high output power was produced.

For example, the communications device 1 may be a mobile transmitterarranged to transmit data using EDGE and GMSK modulation schemes. It mayalternatively be a mobile, or cellular, telephone, or any othertransmitter device. The power controller 4 is arranged to use the secondcontrol loop 22 for the transmission of GMSK modulated signals at a highpower, operating the power amplifier 3 in a non-linear mode forefficiency reasons. The first control loop 21 is used for thetransmission of EDGE modulated signals and also for GMSK modulatedsignals in certain circumstances.

When the first control loop 21 is used, the switch 12 is open during thecentral portion of a time slot during which data is transmitted asdescribed above, and the power amplifier 3 is controlled using thesample and hold circuit 9, 10. As the power detector 14 is disconnectedfrom the variable gain amplifier 11 by the sample and hold circuit 9,10, its output has no effect on the control signal applied to thevariable gain amplifier 11. Therefore, any change in the temperature ofthe power detector 14 does not affect the control and, therefore, theoutput power of the power amplifier 3.

When data is to be transmitted in a GMSK time slot, the controller 20determines whether to operate the power amplifier 3 in a linear ornon-linear mode. Normally, the non-linear mode will be selected by thecontroller 20. When the output power, though, is low and the outputpower in an immediately preceding time slot was high, the controller 20controls the power controller 4 to use the first control loop 21 andcontrols the power amplifier 3 to operate in linear mode.

The efficiency of the power amplifier 3 when operated in linear mode,using the first control loop 21, is lower than when operated innon-linear mode, using the second control loop 22. However, the loss inefficiency is offset by the removal of the drift in the output powercaused by temperature changes in the power detector 14.

The power controller 4 may be arranged so that the power amplifier 3 isoperated in linear mode during a time slot in which non-amplitudemodulated signals are to be transmitted at any power level that is lowerthan an output power used during an immediately preceding time slot.Alternatively, the power controller 4 may determine whether the outputpower to be provided during a time slot is “low” using a predeterminedpower threshold. For example, the threshold may be defined in terms of apower level, or alternatively as a predetermined fraction of the maximumoutput power that can be provided by the power amplifier 3.

Because of the power inefficiencies associated with operating the poweramplifier 3 in linear mode, advantageously the linear mode is used forGMSK time slots only when beneficial. Since there is only significantcooling of the power detector 14 over the duration of a time slot whenthe output power in the time slot is low and the output power in thepreceding timeslot is high, the linear mode preferably is used only insuch circumstances. The exact scheme for determining whether the poweris high or low may be implementation dependent. In particular, theextent of heating and cooling of the power detector 14 may depend oncertain component choices. It is preferred that the linear mode is usedfor GMSK time slots only when the power is significantly lower than inthe immediately preceding time slot. This may be determined by comparingthe ratios of the output power between the time slots to a predeterminedthreshold, or in any other suitable way.

The invention may be implemented in Digital Signal Processing softwareconfigured to control the transmitter 2.

While the embodiments have been described above in relation to acommunication device, such as a mobile telephone, an amplifierarrangement according to the present invention may be used in any datatransmitting device, portable or fixed, including base stationtransmitters, and is particularly suitable for mixed mode datatransmission devices.

In addition, the invention is not limited to amplifier arrangementscomprising the control loops shown in FIG. 2. In particular, it is notessential for the power detector 14 to be located within the poweramplifier 3.

1. A transmitter arranged to transmit data modulated using one of afirst modulation scheme and a second modulation scheme in each of aseries of time slots, the transmitter comprising: a power amplifieroperable in linear and non-linear modes; power control means, arrangedto control the power supplied by the power amplifier; and a controllerarranged to control the power amplifier to operate in the linear mode intime slots when the first modulation scheme is applied, and to operatein the non-linear mode in time slots when the second modulation schemeis applied, the controller being arranged, in respect of a time slot inwhich the second modulation scheme is used, to determine whether atransmission power in the time slot is lower than the transmission powerin an immediately preceding time slot, and in response to a positivedetermination to control the power amplifier to operate in the linearmode during the time slot.
 2. A transmitter according to claim 1,including a sample and hold circuit operable to maintain the gain of thepower amplifier at a generally constant value between ramp up and rampdown portions of a time slot when operating in a linear mode.
 3. Atransmitter arranged to transmit data modulated using one of a firstmodulation scheme and a second modulation scheme in each of a series oftime slots, the transmitter comprising: a power amplifier operable inlinear and non-linear modes, the power amplifier comprising first andsecond amplifier stages connected in series; power control means,arranged to control the power supplied by the power amplifier, the powercontrol means including first and second control loops; and a controllerarranged to control the power amplifier to operate in the linear mode intime slots when the first modulation scheme is applied, and to operatein the non-linear mode in time slots when the second modulation schemeis applied, wherein the controller is arranged, in respect of a firsttime slot in which the second modulation scheme is used, to determinewhether a transmission power in the first time slot is lower than thetransmission power in a second time slot, the second time slotimmediately preceding the first time slot, and in response to a positiveresult to the determination to control the power amplifier to operate inthe linear mode during the first time slot, in which in the non-linearmode the first amplifier stage is controlled to have a fixed gain andthe second amplifier stage is controlled to have a gain dependent on asecond amplifier stage control signal generated by the first controlloop, and in which in the linear mode the second amplifier stage iscontrolled to have a fixed gain and the first amplifier stage iscontrolled to have a gain dependent on a first amplifier stage controlsignal generated by the second control loop.
 4. A transmitter accordingto claim 3, in which the second control loop includes a sample and holdcircuit operable to maintain the first amplifier stage control signal ata generally constant value between ramp up and ramp down portions of atime slot.
 5. A transmitter according to claim 1, comprising a powerdetector integrated with the power amplifier.
 6. A communication deviceincluding a transmitter as claimed in claim
 1. 7. A radiotelephoneincluding a transmitter as claimed in claim
 1. 8. A transmitteraccording to claim 3, comprising a power detector integrated with thepower amplifier.
 9. A communication device including a transmitter asclaimed in claim
 3. 10. A radiotelephone including a transmitter asclaimed in claim
 3. 11. A method of operating a transmitter, the methodcomprising: transmitting data in a series of time slots, applying one ofa first modulation scheme and a second modulation scheme to data in eachtime slot; and controlling a power amplifier to operate in a linear modein time slots when the first modulation scheme is applied, and tooperate in a non-linear mode in time slots when the second modulationscheme is applied, the method further comprising, in respect of a timeslot in which the second modulation scheme is used: determining whethera transmission power in the time slot is lower than a transmission powerin an immediately preceding time slot, and in response to a positiveresult to the determination, controlling the power amplifier to operatein the linear mode during the time slot.